Ram jet control system



March 27, 1962 M. OLLODORT RAM JET CONTROL SYSTEM 2 Sheets-Sheet 1 Filed Aug. 21, 1956 r R. .6 mo 5 N0 mu 2 W m w? m M a N w w, y

March 27, 1962 Filed Aug. 21, 1956 M. OLLODORT RAM JET CONTROL SYSTEM 2 Sheets-Sheet 2 C7 (0. roe

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c supze 62/77:, I L 51 I IM 1 Mt Mmw zvu/vaez INVENTOR. Made/5 all 006 67 United States Patent Ofiice 3,026,573 Patented Mar. 1 96.2..

3,026,673 RAM JET CONTROL SYSTEM Morris Ollodort, Whittier, Caiif., assignor to the United States of America as represented by the Secretary of the Air Force Filed Aug. 21, 1956, Ser. No. 605,450

4 Claims. (Cl. oil-35.6)

This invention relates to a fuel control system, and more particularly, to a ram jet fuel control system for controlling the flow of fuel to a ram jet engine so that the maximum possible ratio of combustion chamber pressure to free stream total pressure is maintained, and to a control system which is provided with means for eliminating ram jet buzz.

In the past, the fuel requirements for combustion in a ram jet engine were thought to vary directly with the square root of the ram pressure created by the forward velocity of the engine. This suggested that the ram pressure might be conveniently and directly utilized in a fuel control system to deliver the required quantity of fuel to the engine at any velocity. When this was done, certain difiiculties occurred because when the altitude at which the engine was operating was increased at a constant rate, the fuel requirements of the engine decreased at a faster rate than the fuel control system based directly on the ram pressure indicated. This caused the control system to supply an unnecessarily rich mixture of fuel to the engine. In addition, at low speeds, the ram pressure available was not suflicient to insure optimum fuel atomization.

Furthermore, when the pressure sensing means used to control the supply of fuel to the engine was mounted in the engine, it was affected by pressure fluctuations in the combustion chamber of the engine. Consequently, the fuel supplied to the engine fluctuated in response to these pressure changes and engine performance suffered. When the pressure sensing means was mounted outside the engine to escape these pressure fluctuations in the engine, certain aerodynamic factors had to be considered. When air, traveling at supersonic velocities, enters into a ram jet engine, its velocity energy is transformed into pressure energy. When this happens, a shock Wave or front normal to the axis of the engine and to the direction of the air flow is set up whereby the air upstream of the shock wave is moving faster than the local speed of sound, while the air downstream of the shock wave is moving at a speed less than the local speed of sound. The position of this shock wave in the engine is partially dependent on the combustion chamber pressure. This means that higher combustion chamber pressurestend to force the shock wave out toward the lip of the engine and lower combustion chamber pressures tend to let the shock wave move downstream. This combustion chamber pressure in turn is affected by the quantity of fuel supplied to the engine, and when the quantity of fuel supplied to the engine was determined directly by the ram pressure, it frequently happened that an increase in ram pressure caused by a dive, for example, resulted in such an increase in fuel supplied to the engine and a consequent increase in combustion chamber pressure in the engine that the normal shock Wave in the engine was forced out beyond the lip of the engine.

7 Whenever this occurs, there is a sudden drop in pressure in the combustion chamber that permits the shock wave to reenter the engine; but as soon as this happens, the combustion chamber pressure suddenly rises. The rise in combustion chamber pressure again forces the shock wave out beyond the lip of the engine. This happens very rapidly, and the oscillatory movement of the shock wave ,at the lip of the engine is known as buzzing. Furthermore, since the combustion chamber pressure fluctuates considerably during engine operation and causes the position of the shock wave in the engine to fluctuate, transient aerodynamic pressure conditions in the com-bus tion chamber may vary so much that the shock wave may be forced out beyond the lip of the engine and cause buzzing even while the engine is operating at design conditions. Therefore, a principal object of this invention is to provide a means for controlling the flow of fuel to a ram jet engine so that the maximum possible practical ratio of combustion chamber pressure to free stream total pressure is maintained in the engine regardless of the speed at which the engine is moving and regardless of transient aerodynamic fluctuations in the engine in order that the engine will operate at its highest practical efficiency.

A further object of this invention is to provide a fuel control system that eliminates ram jet buzz due to the vibratory motion of the supersonic normal shock wave at the lip of the engine.

An additional object of this invention is to correlate the supply of fuel delivered to the engine with engine requirements regardless of the altitude.

Still another object of this invention is to provide a means for conveniently altering the behavior of the fuel control system in a ram jet engine so that a standard ram 'jet engine can be used in situations requiring different performance characteristics.

Other objects and advantages of this invention will be readily understood from the following detailed description of a typical preferred embodiment of the invention wherein reference will be made to the accompanying drawings in which,

FIG. 1 is a schematic diagram of the ram jet fuel control system.

FIG. 2 is an enlarged sectional view of a ram jet engine showing the location of the normal shock wave in the interior of the engine, air flow lines, and the position of the static pressure probe and the combustion chamber pressure probe in the engine. I FIG. 3 is an enlarged sectional view of the ram jet engine showing how in subcritical operation the movement of the normal shock wave forward of the lip of the engine intersects the oblique shock wave and interferes with the flow of air into the engine.

FIG. 4 is a diagram illustrating the relationship be tween the thrust coeflicient and the mach number for a fixed geometry ram jet engine.

. Referring now to FIG. 1, a ram jet engine 12 has a fuel control system indicated generally at 10. A standard difiuser 14 functioning as an analogue is disposed in spaced parallel relationship to the engine 12. The ram jet engine 12 has a center body 16 and includes a diffuser portion 18, a diffuser throat i9 and a combustion chamber portion 20. The standard diffuser 14 comprises an outer shell 22 and a center body 24. A combustion chamber pressure probe 26 which provides a measure of the entrance pressure P in the combustion chamber 20 is positioned at the point of maximum pressure recovery in the ram jet engine. This point in each engine is determined by design considerations, taking into account the geometry of the engine and the altitude and speed at which the engine is intended to operate. The mathematical significance of the maximum pressure recovery is as follows: Referring to FIG. 2, if P is the free stream total pressure and P is the pressure at some point downstream of the normal shock wave 23 in the engine, then aerodynamic theory shows that other variables being held constant; and the value of P 3 for which the derivative F(P is the value of the pressure at the point of maximum recovery. The ratio 1 is a measure of the diffuser efficiency. So if an engine is designed so that the combustion chamber pressure is at the point of maximum pressure recovery, or in other words, so that the ratio between the combustion chamber pressure to the free stream total pressure is a maximum, the engine will operate at its greatest theoretical efficiency.

A ram air pressure probe 28 detecting pressure P is positioned in the standard diffuser 14 substantially at the normal shock. This point is likewise dictated by the geometry of the diffuser and by aerodynamic considerations. This ram air pressure probe 28 is used as a reference pressure, and because the standard diffuser 14 is disposed in spaced parallel relation to the engine, it has none of the disturbing complications caused by the presence of pressure fluctuations from the combustion chamber 20 in the engine 12. A conduit 30 connects pressure probe 26 to bellows 32 and conduit 34 connects pressure probe 28 to bellows 36. One end of bellows 32 and one end of bellows 36 are secured to a stationary support, see FIG. 1. The opposite end 33 of the bellows 32 is connected to a pivotally mounted switch bar 40 at a distance d from the pivot 38 and the opposite end 35 of bellows 36 is connected to the pivotally mounted switch bar 40 at a distance d from the pivot 38. The connections of bellows 32 and 36 to switch bar 40 are on the opposite sides of the pivot 38 and produce a differential pressure responsive switch 42. This differential pressure responsive switch 42 compares the relatively variable combustion chamber pressure P detected by pressure probe 26 in the engine 12, with the comparatively stable ram pressure P detected by the pressure probe 28 in the standard diffuser 14. The differential pressure responsive switch 42 is designed so that when the combustion chamber pressure P in the ram jet engine 12 is at its designed value, switch bar 40 in the differential pressure responsive switch 42 will remain in neutral position, see FIG. 1. When the pressure in the combustion chamber departs from this design value causing the position of the shock wave in the engine to vary, the balance in the differential pressure responsive switch 42 is destroyed and the switch bar 40 will abut contacts 46 or 48 of the motor driven valve 50 to increase or decrease the supply of fuel to the engine by changing the size of orifice S1 in the engine fuel line 52. The purpose of increasing or decreasing the fuel supply to the engine is to restore the combustion chamber pressure to its designed value and to force the normal shock wave back to its proper position in the engine adjacent the diffuser throat 19. This is possible because the position of the normal shock wave in the engine is dependent upon the combustion chamber pressure and this pressure in turn is dependent upon the fuel supply to the engine.

Assuming the bellows 32 and 36 have identical cross sectional areas A, when the differential switch 42 is at equilibrium, then referring to FIG. 1,

P XAXd1=P XAXd2 or cancelling out A Now aerodynamic theory requires that if P is the pressure at the altitude the engine is flying and if M refers to the Mach number, then P =P F (M) and P will equal P XF JM), other variables being held constant. Therefore 4 and cancelling out P the altitude pressure, we have Pn 5 M) H F JM) but remembering which can be set equal to a constant K. In other words, this shows that the operation of this fuel control system is independent of the altitude.

It sometimes happens that pressure fluctuations in the combustion chamber of the engine are so severe that the normal shock wave 23 is forced out beyond the lip 56 of the engine before the fuel control system actuating the differential pressure responsive switch 42 can be effective. When this happens, the engine operates subcritically and buzzing occurs. The fuel control system governed by the differential pressure responsive switch 42 acts only to bring the normal shock wave 23 in the engine into a location such that When buzzing occurs, the normal shock wave 23 moves upstream beyond the lip 56 of the engine and intersects the oblique shock wave 25 formed at the tip of the center body 16, see FIGS. 2 and 3. When this happens, the air stream to the engine is disturbed and the amount of air entering the engine is decreased, see FIG. 3. This causes the overall value of P to drop. If the fuel control system should increase the fuel to air ratio and hence the combustion chamber pressure, equilibrium would not be restored since an increase in P would prevent the reentry of a normal shock wave into the engine. To prevent this from happening, a special buzz control system is necessary.

This buzz control system comprises a static pressure probe 54 positioned adjacent lip 56 of the ram jet engine 12. This static pressure probe 54 is connected to a conduit 50 which transmits the static pressure P, to bellows 60 in a second differential pressure actuated switch 70. An additional conduit 62 connects the diffuser probe 28 with bellows 64. One end each of bellows 60 and bellows 64 are connected to a stationary support. The opposite end 61 of bellows 60 and end 65 of bellows 64 are connected to a movable switch bar 66 on opposite sides of pivot 68. The static pressure P in the ram jet engine acting on switch bar 66 through bellows 60 and the ram air pressure P acting on the rod 66 through bellows 64 oppose each other, and as in the case of the first differential pressure actuated switch 42, the effects of the altitude are cancelled. When the diffuser portion 18 on the ram jet engine 12 is acting supercritically, i.e., the normal shock wave is downstream of the static pressure probe, the differential pressure responsive switch 70 is designed so that switch lever 66 does not abut contact 69 which operates a solenoid valve 72. This permits the fuel to flow in fuel line 52 past the solenoid valve 72 without interference. Aerodynamic theory shows that as the shock wave 23 in the engine approaches a static pressure probe, the static pressure P, detected by the probe increases very sharply. Therefore, when conditions in the engine tend toward subcritical operation, i.e., when the shock wave 23 moves upstream of the engine toward the engine lip 56 and toward the static pressure probe 54, the static pressure probe 54 will sense a large increase in pressure. This is communicated to bellows 60 and forces the switch lever 66 against contact 69 of solenoid valve 72. When this happens, the valve 72 closes and the fuel is forced to bypass the solenoid valve 72 and passes through a narrow conduit 74. This restricts the flow of fuel into the engine and decreases the combustion chamber pressure. This drop in pressure permits the normal shock wave to move back into the engine where the first differential pressure. actuated switch 42 again becomes effective to restore the combustion chamber pressure P to its designed value. The entire result of this operation is that the engine is not permitted to operate subcritically and buzzing cannot occur. It is, of course, within the contemplation of this invention that the differential pressure responsive switches shown in this case can be replaced by various electrical circuits such as Wheatstone bridges, for example, to accomplish the same result.

The advantages of using a standard diffuser as an analogue for the control system is apparent by consideration of FIG. 4. This is a diagram showing the effect of an increasing mach number on the thrust coelncient of the invention. As can be seen, the geometry of the engine and the conditions under which it operates, dictate a critical line. Since the thrust coefl'icient increases with the amount of fuel supplied to the engine, at a particular mach number, the fuel actually supplied to the engine may increase the value of the thrust coefficient to a point above the critical line. When this occurs, the engine will operate subcritically and buzzing occurs. Physically, this is when the normal shock wave enters into a vibratory action around the lip of the engine. The region of the diagram above the critical line is called the subscritical region. The region below the critical line is called the supercritical region. The best line of operation of a ram jet engine from the standpoint of performance is along the critical line. However, an operational margin is required, be cause even minor fluctuations in the engine, and in particular in the pressure in the combustion chamber may drive the engine into subcritical operation. This possibility may be avoided by designing the fuel control system so the engine operates on an optimum engine operational line, which is parallel to the critical line, see FIG. 4. This has the advantage that subcritical operation can be avoided since the operational line never intersects the critical line. Noting, however, the position of the thrust coeflicient line required for level flight and considering that ram jet operations do largely occur at level flight, it can be seen that if conditions should cause the mach number to abruptly decrease away from the design value at point A on the diagram to a point B on the diagram, while following the optimum engine operational level, the difference AC between the thrust coeflicient required, see point C on the diagram, and the thrust coeflicient available, see point B on the diagram, would be very large. Therefore, to maintain operation in such a case, a steep dive to increase the mach number and restore the proper value of the thrust coefficient would be required. If the fuel system were designed to cause the engine to follow the level flight line, it can be seen that a decrease in mach number from the design value of the mach number M at point A on the diagram, to mach number value M at point C on the diagram, would cause subcritical operation. At any rate, taking into consideration the various possibilities, some compromise line is determined in accordance with the mission of the engine. This compromise line determines the operation of the fuel control system. Since the standard diffuser is used to provide a reference pressure for the control system, its shape and performance can be modified to cause the fuel control system to follow any desired compromise line, and in this kind of a fuel control it enables the proper quantity of fuel to be delivered to the engine and accomplishes this without being affected by transient pressure fluctuations in the engine.

Since the operation of the fuel control system and the performance of the engine is affected by the shape and design of the standard diffuser, this suggests that a standard ram jet engine could be used with a variety of differently shaped standard diffusers to accomplish many desired engine missions. In other words, this fuel control system suggests the possibility of utilizing just one standard ram jet engine with a plurality of differently shaped standard diffusers to accomplish certain missions to take the place of a larger number of separately designed ram jet engines to accomplish these same missions.

Having thus described the invention, what is claimed as new to be secured by Letters Patent is:

1. An apparatus of the class described comprising in combination a ram jet engine, buzz control means for the ram jet engine, said buzz control means comprising static air pressure sensing means in said ram jet engine, an analogue, ram air pressure sensing means in said analogue, pressure comparing means connected to said static air pressure sensing means in said ram jet engine, and to said ram air pressure sensing means in said analogue for comparing static air pressure in the ram jet engine with the ram air pressure in the analogue, fuel valve means connected to the ram jet engine for restricting the flow of fuel to the ram jet engine, said fuel valve means connected to and controlled by said pressure comparing means and operating to restrict the flow of fuel to the ram jet engine when the static air pressure in the ram jet engine exceeds the ram air pressure in the analogue by a predetermined amount, and means for controlling the flow of fuel to the ram jet engine so that the maximum possible ratio of combustion chamber pressure to free stream total pressure is maintained.

2. The apparatus set forth in claim 1 wherein said comparing means comprises a differential pressure responsive switch, said differential pressure responsive switch connected to said valve means so that when the static air pressure in the ram jet engine exceeds the ram air pressure on the analogue by a predetermined amount said dilferential pressure responsive switch will actuate said valve means to restrict the flow of fuel to the engine to prevent buzzing.

3. An apparatus of the class described comprising in combination a ram jet engine and an analogue, combustion chamber pressure sensing means in said ram jet engine, ram air pressure sensing means in said analogue, a first comparing means connected to said combustion chamber pressure sensing means in said ram jet engine and to said ram air pressure sensing means in said analogue, fuel metering means connected to said ram jet engine to vary the flow of fuel to the engine, said fuel metering means connected to and controlled by said first comparing means and operating to increase or decrease the fuel supply to the engine in accordance with the difference between the combustion chamber pressure in the ram jet engine and the ram air pressure in the analogue so that the maximum possible ratio of combustion chamber pressure to free stream total pressure is maintained, static air pressure sensing means in the ram jet engine, a second comparing means connected to said static air pressure sensing means in said ram jet engine and to said ram air pressure sensing means in said analogue, fuel valve means connected to said ram jet engine, said fuel valve means connected to and controlled by said second comparing means so that when the static air pressure in the ram jet engine exceeds the ram air pressure in the analogue a predetermined amount, said second comparing means will actuate said valve means to restrict the flow of fuel to the engine to prevent buzzing.

4. An apparatus of the class described comprising in combination a ram jet engine and a diffuser disposed in spaced parallel relation to said ram jet engine, combustion chamber pressure sensing means in said ram jet engine, ram air pressure sensing means in said diffuser, a first differential pressure responsive switch connected to said combustion chamber pressure sensing means in said ram jet engine and to said ram air pressure sensing means in said diffuser, a motor driven valve connected to said ram jet engine to vary the flow of fuel to the engine, said motor driven valve connected to and controlled by said differential pressure responsive switch and operating to increase or decrease the fuel supplied to the engine in accordance with the pressure diiference between the combustion chamber pressure in the ram jet engine and the ram air pressure in the diffuser, so that the maximum sensing means inthe ram jet engine, a second dificrential pressure responsive switch connected to the static air pressure sensing means in the ram jet engine and to said ram air pressure sensing means in said difiuser, fuel valve means connected to said ram jet engine for restricting the flow of fuel to the ram jet engine, said fuel valve means connected to and controlled by said second pressure responsive switch so that when the static air pressure in the ram jet engine exceeds the ram air pressure in the diffuser a predetermined amount said diflerential pressure responsive switch will actuate said fuel valve means to restrict the flow of fuel to the engine and prevent buzzing.

References Cited in the file of this patent UNITED STATES PATENTS 2,706,885 Ostrofi et al. Apr. 26, 1955 2,766,579 Gallo et al. Oct. 16, 1956 2,855,753 McLatfcrty Oct. 14, 1958 2,880,579 Harshman Apr. 7, 1959 2,914,911 Richter Dec. 1, 1959 

